This invention relates, in general, to pressure transducers and, more particularly, to pressure senders for providing a predetermined electrical resistance in response to a particular sensed pressure.
Many applications require that fluid pressure in a system be monitored. For example, it is often desired to provide in automobiles gauges which quantitatively indicate oil pressure in lieu of the usual indicator light which merely indicates a "zero-pressure" condition.
Generally, such pressure gauges are electrically connected in series to what are generally termed pressure senders. Essentially, such a pressure sender includes a variable resistor assembly (in the gauge circuit) and pressure sensing apparatus adapted to be tapped into the line in which the fluid whose pressure is to be measured is carried. The resistance presented by the sender in the gauge circuit is a function of the sensed pressure. Since a variation in the pressure in the line will cause a responsive variation in the resistance of the sender, the reading of the gauge, which is serially connected to the sender, will reflect the fluid pressure sensed.
Most presently existing pressure senders generally include a fixed resistor element, which may comprise a wound coil, a pressure sensing diaphragm which fluidly communicates with the pressure line and a contact which is moved into electrical engagement with the resistor to an extent determined by the sensed pressure so that a pressure resistance is obtained from the coil for a particular sensed pressure. The coil and contact are serially connected to the gauge circuit and, therefore, a variation in resistance of the coil will cause a variation in the gauge reading. However, these existing pressure senders are not entirely satisfactory for a variety of reasons. Often, the sensitivity of such senders is insufficient to provide a well-defined "cut-off" pressure, i.e., a minimum pressure below which, a zero reading (open circuit) on the gauge will result. Closely associated with this problem is that currently available pressure senders are often difficult to precisely calibrate due to the senders' relative insensitivity at the relatively low pressures used during calibration.
Another problem inherent in prior art pressure senders is that of hysteresis. More particularly, it has been found that identical pressures in the pressure line will not produce identical resistances in the sender (and, thus, will not result in identical readings by the gauge) after repeated cycling of the pressures. This has been found to be due to the fact that the contact, under the sender being subject to a pressure previously sensed, does not move into the same position (to produce the same resistance as it did originally, due to the relatively low restoring forces available.
Yet another serious problem is in obtaining pressure senders in quantity production which have substantially identical sensitivity during operation. These problems have generally resulted from the manufacturing tolerances inherent in large-scale manufacturing operations.